Contrast detection focusing technology is one of widely used passive auto focus technologies at present. The principle of the contrast detection focusing technology is that, according to a contrast data change of sampling images at different focus depth positions of a photographing device, a focus depth position corresponding to the maximum contrast data is taken as the optimal focus depth position, that is, a in focus depth position.
For example, when a contrast of the sampling image is used as contrast data, a contrast detection focus process is usually as follows: move a focusing lens along a depth direction; with the movement of the focusing lens, a sampling image acquired at an imaging plane of an image sensor gradually becomes clear and the contrast gradually increases as well; when the sampling image is the clearest, and the contrast is the highest, the imaging plane of the image sensor is at the in focus depth position, which, however, is not known by a photographing device; the photographing device continues to move the lens along the depth direction; when the contrast of the sampling image is found to decrease, it is determined that the photographing device misses the in focus depth position, so the lens is moved in a reverse direction. Such an adjustment is repeated many times, until a position of the imaging plane of the image sensor approaches or coincides with the in focus depth position to the maximum extent. It may be seen that the foregoing contrast detection focus process needs multiple times of “missing” the in focus depth position to achieve auto focus, and a focusing speed is comparatively low.